Dear B. Woods:
Thank you for writing back to me. I hope that you and your family are well. I have been very disappointed with pool contractors that do not go to the national pool shows, manufacturers sponsored seminars, trade magazines etc., and are still using technology from 30 years ago. There are so many great new products like salt generators, lighting, water features etc. You can only heat one body of water at a time. So when you have a spa that overflows into the pool, when you want to go in the spa you either manually turn two valves to isolate the spa so that it doesn't overflow into the pool or you push a button and the valves turn automatically to stop the waterfall and just circulate water in the spa. So with two thermostats you keep your favorite temperature for the pool and your favorite temperature for the spa. Then on some heaters when you switch to spa the thermostats switch automatically from pool to spa. So when you are done heating the spa you turn the valves back to pool and you heat the pool again. The size of the heater is really dependent on how fast you want the spa to heat up. A 100,000 BTU heater will heat a 500 gallon spa about 18 degrees and hour when it is warm out. (Please no letters if readers are not getting that much as it depends on many things like wind, RH, outdoor air temp, ground temp, piping run etc.) So most people use one heater for the spa and the pool. Some use two heaters also. If you are planning to use the spa in the winter and it is below 50 degrees outside depending on which you decide on, heater then it won't work. You will need a special heat pump like the Jandy AE 2500 TR with the hot freon defrost feature to go in very cold weather.
I hope that I answered all your questions.
360 568 3718